1. Field of the Invention
This invention relates to an ArF excimer laser device, and more particularly an ArF excimer laser device having a narrow spectral line width for exposing a semiconductor.
2. Description of the Related Art
With semiconductor integrated circuits becoming more and more fine and highly integrated it has become necessary to improve the resolution capacity of a projection exposure device. Accordingly, it has been aimed at obtaining exposure light having a short wavelength to be radiated from the light source for the exposure, and an ArF excimer laser device has become important as a light source for exposure of next generation semiconductors.
In the ArF excimer laser device, laser gas acting as the laser medium is excited by generating an electrical discharge within a laser chamber in which, as the laser gas, a gas mixture composed of fluorine (F2) gas, argon (Ar) gas and rare gases such as neon (Ne) gas acting as a buffer gas is contained at several 100 kPa.
In addition, since the spectral line width of the laser beam emitted from the ArF excimer laser device is wide (about 400 pm) it has become necessary to narrow the spectral line width as much as possible in order to avoid the problem of aberration in the projection optical system of the exposure device. In the prior art, this narrow spectral line width is obtained by a method wherein an optical system for narrowing the line width which is composed of a beam expansion prism system and a diffraction grating, for example, is installed within the laser resonator.
When, under these circumstances, the ArF excimer laser device is installed in an ArF scanner-type stepper device for use in the exposure of semiconductors, the requirement of narrowing the line width is even more severe as a high NA (numerical aperture) of the exposure device is realized to increase the resolution. In the case that NA is 0.6 or more it is normally required to attain an oscillation wavelength having a half width (full width at half maximum) of 0.5 pm or less. Further, more importantly, it becomes necessary to have a width containing 95% of the energy (95% of the integrated line width) of 1.15 pm. Further, in the case that NA is 0.7 or more, it is required to have a super-narrow band with a half width of 0.35 pm and a width containing 95% of the energy of 0.85 pm or less.
As described above, in the prior art methods for narrowing the line width a laser beam is expanded through a prism system and the laser beam is processed in a spectroscopic manner through an echelle diffraction grating. However, such a method as above does not satisfy the aforesaid requirement. In addition, although it has also been proposed to provide the method for using a narrow bandwidth in combination with an element for forming a narrow bandwidth as an etalon or the like, there remain the problems that the lifetime of the element for narrowing the line width such as an etalon or the like is short and its control is quite difficult.
The present invention has been made with reference to the problems of the prior art as described above, and it is the object of the invention to devise conditions for obtaining a spectral bandwidth containing 95% of the energy (integrated width beam by 95%) as narrow as 1.15 pm or less when using an optical system for narrowing the line width comprised of a prior art beam expansion prism system and a diffraction grating and to devise an ArF excimer laser device having a narrow bandwidth for use in semiconductor exposure.
The ArF excimer laser device for narrowing the bandwidth of the present invention solving the aforesaid object has a line-narrowing optical system comprised of an echelle diffraction grating of the Littrow arrangement, a beam expansion prism system composed of three prisms arranged at the incident side of the echelle diffraction grating, and slits, and the blaze angle xcex8 of the echelle diffraction grating is 82 or less, the magnification rate M of the beam expansion prism system is 26 times or less, the oscillation pulse width Tis is 60 ns or less, the length L of the resonator is in a range of 1000 to 1350 mm and the slit width W is 1.0 mm or more, wherein:
(W+11) cos xcex8/(LMTi80.853) less than 4.94xc3x9710xe2x88x926xe2x80x83xe2x80x83(14) 
Another ArF excimer laser device for narrowing the bandwidth of the present invention has an optical system for narrowing the bandwidth comprised of an echelle diffraction grating having a Littrow arrangement, a beam expansion prism system composed of three prisms arranged at the incident side of the echelle diffraction grating, and slits, and the blaze angle xcex8 of the echelle diffraction grating is 82xc2x0 or less, the magnification rate M of the beam expansion prisms is 26 times or less, the oscillation pulse width Tis is 60 ns or less, the length L of the resonator is in a range of 1000 to 1350 mm, and the slit width W is 1.0 mm or more, wherein:
(W+11) cos xcex8/(LMTi80.853) less than 3.65xc3x9710xe2x88x926xe2x80x83xe2x80x83(15) 
Also in these cases, it is possible to set the repetition rate to more than 3 kHz.
In the present invention, where either the equation (14) or the equation (15) is satisfied, an ArF excimer laser device having either a 95% integrated line width of 1.15 pm or 0.85 pm can be realized, which has been assumed to be quite difficult up to now, and its output can be used in connection with an optical system for narrowing the bandwidth comprised of a prior art beam expansion prism system and a diffraction grating.